Magnetic Micro-Origami

Grain coalescence: happen when the mass of two neigh- boring grains differ significantly, the larger grain incorporates the smaller one while the total number of grains is reduced by one.

Heteroepitaxy: Heteroepitaxy is the growth of a crystalline film on a crystalline substrate of a different material.

Hygroscopicity: a physical property of a material, is a measure of how well the material can absorb and release water molecules

Epitaxy: refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer.

Indium arsenide atoms arriving on the AlAs surface must reduce their interatomic distances to match the lattice of AlAs. Therefore, the InAs film on AlAs is in compressed state. On the other hand, GaAs atoms experience tensile stresses when they form single crystal film on the top of InAs with larger lattice constant. When AlAs sacrificial layer is selectively etched (with selectivity better than 1:1000), the initially com‐ pressed InAs expands and the strained GaAs shrinks resulting in bending and rolling of the pattern of the bilayered film of InAs/GaAs which eventually forms a multiwall tube. For very thin films (a few atomic layers), the 7.5% mismatch strain between InAs and GaAs lattices results in a very small diameter of the tubes (down to 2 nm). Depending on the size of the rectangular pattern the number of turns can change between 1 and 40. The diameter of the tubes can be designed using Formula 1. Here, it is important to note that ferromagnetic films such as single crystalline Fe or antiferromagnetic films such as , , or can be grown epitaxially on top of GaAs.

A concept of using anisotropic properties of composite materials to control rolling direction

(a) Al/Pt film pattern deposited on top of photoresist and (b) the same bilayered film deposited on top of glucose film (images obtained by optical profiler).

First, the 50 μm × 50 μm holes were defined in a PMMA photoresist using exposure of the photoresist to deep UV light through as mask. Thin film of Cu was deposited by sputtering on the Si wafer with (001) orientation. During a lift-off process the Cu film pieces on top of photoresist were removed by ultrasonication of the wafer immersed in acetone. The remaining 50 nm thick Cu squares served as sacrificial layer. Another mask with 20 μm × 50 μm rectangles was used to deposit the trilayered Ti/GaFe/Au and Ti/Ni/Au films. This mask was aligned in such a way that the majority of the area of the rectangular holes in photoresist was on top of the Cu squares and a smaller part on exposed Si. After deposition of the trilayer and the second lift-off process the rectangles remained on top of Cu patches, which were partially attached to Si, as presented in Figure 4(a). During selective etching of the sacrificial Cu underlayer, the films with residual stresses, caused by the grain coalescence, self-rolled, and formed magnetic microtubes with 3–4 turns, diameter of 5–10 μm, and the length of 20 μm Figure 4(b)

Magnetization hysteresis loops of rectangular film patterns 20 μm × 50 μm with Ni layer (black line), micro‐ tubes (red line), and anodized microtubes (blue line). Top part refers to the 0° angle and the bottom to 90° angle of the applied magnetic field

Magnetization hysteresis loops of rectangular film patterns 20 μm × 50 μm with GaFe layer (black line), mi‐ crotubes (red line), and anodized microtubes (blue line). Top part refers to the 0° angle and the bottom to 90° angle of the applied magnetic field.

It is possible to fabricate origami figures with features as small as 2nm.

Adhesion forces, surface tension, or interfacial stresses start playing important roles when the size of the figures decreases below several micrometers.

Taken from introduction: Magnetostrictive and multiferroic materials take a special place among the stimuli-responsive materials, because they couple magnetic, elastic, and electronic properties of the materials.

Taken from surface tension: Self-assembled structures can serve as building blocks for designing mesoscopic metamaterials with interesting responses to electromagnetic radiation in microwave, millimeter wave, and optical ranges

• Magnetic films are good candidates for the micro- and nano-origami structures.
• Forces:
  Surface tension
  This force is of primary importance in micro- and nanoscales. The surface tension of evaporating drop of liquid s sufficient to pull the walls of the photolithographically defined thin film patterns together. These patterns typically consist of more rigid areas connected by thinner areas. In some cases, the capillary forces can produce reversible conformations due to swelling and dehydrating of porous structures.

Residual and interfacial stresses
Growth of polycrystaline films may lead to significant residual stresses due to coalescence of grains. The curvature of the films depends on the internal stress level and thickness of the film. According to Equation 1, the smaller origami figures the tinner films must be used. If the tensile stresses in the bilayers are smaller near the substrate than o the film surface, the film released from the substrate tends to bend under and may form wrinkles. Much better control of stresses can be achieved in heteroepitaxial structures.
They used misfit between single crystalline semiconductor films grown epitaxially on top of each other as shown in Figure 1 (I did not understand a thing).
Thermal stresses in thin films provide another way to deform flat patterns after release from the substrate.

• Other useful forces
  Application of external stresses can be realized by deposition of the films on bent or strained substrates.
  Significant stresses in thin films can occur when the films undergo change of crystal structure or are chemically altered.

• Controlling direction of self-assembly:
  The shape may define the direction. Films with uniform in-plane stresses is proportional to the length of the edge. The bending of longer edges prevails. Rigid parts of the pattern, such as thicker walls of origami can act as constraints. For certain stress gradient level a regular wrinkling is expected rather than bending. Control of rolling directions is easier to achieve in heteroepitaxial structures.
  Anisotropic elastic properties in combination with selective anisotropic etching can be used to fabricate variety of three-dimensional structures by depositing film patterns at different angles.
  Aligned carbon nanotubes embedded in polymer or magnetic film can control the pitch of the helical structures depending on the angle between the pattern and the direction of the enforcing element as shown in Figure 2.

• A sacrificial layer
  Vast majority of techniques uses solid sacrificial underlayers to build patterns on top of them.
  Requirementes:
  The sacrificial layer must form a smooth surface, promote proper growth of the film, and enable patterning. The microstructure of the polycrystalline films may strongly depend on the type of a substrate. An example shown in Figure 3.
  The sacrificial layer must be from the material which has markedly different etching properties than the film pattern. (What does this mean?)
  The sacrificial layer must also be a part of the heteroepitaxial system. This means that it must match both the structure of the substrate and the films that are grown on top of it.
  NaCl is a good candidate to grow magnetic films with cubic crystal structure. Not so often used because of its higroscopicity.
  Procedure for the electron beam evaporation to grow heteroepitaxial structures with NaCl sacrificial layer:

  • A thin layer of MgO (typically nm) was deposited on MgO substrate to improve the quality of the surface.
  • NaCl was evaporated at the rate of 0.05 at 350 C
  • Various magnetic and nonmagnetic transition metal films, such as Fe, Cr, V and Ag, were grown epitaxially on top on NaCl.
    This layer dissolves in water.
    Zn and Mg are interesting materials for the sacrificial layers because they sublimate in vacuum at relatively low temperatures

• Link between the shape and magnetic properties
  Angular deposition of Au/Co/Au films resulted in the in-plane magnetic anisotropy of rectangular film patterns. Also changes were observed in the shape of the hysteresis loop and evolution of magnetic domain structure when the film patterns rolled and formed microtubes.
  The hysteresis loops of the microtubes with Ni are presented in Figure 5. The magnetic field was applied in two transverse directions. The direction 0 is the direction of the axis of the tubes formed from the patterns and the 90 is transverse to the tubes.

The behavior of the GaFe films, Figure 6, could be interpreted in terms of the shape anisotropy of the microtubes for which the technical magnetization saturation is achieved at lower fields for the field applied along the easy magnetization axis, whereas the loops for the hard direction are tilted and approach saturation magnetization at higher fields. Both type of materials exhibit opposite trends in the change of the coercive field. The coercivity of Ni tubes almost triples for the field applied at 90 while it decreases by about 30% for GaFe films. These facts prove that the change of shape and associated shape anisotropy alone is unable to explain the changes of magnetic properties.